Department of Systems Biology, Harvard Medical School and Marine Biological Laboratory, Woods Hole, Massachusetts, USA.
Cytoskeleton (Hoboken). 2012 Oct;69(10):738-50. doi: 10.1002/cm.21050. Epub 2012 Aug 20.
Ray Rappaport spent many years studying microtubule asters, and how they induce cleavage furrows. Here, we review recent progress on aster structure and dynamics in zygotes and early blastomeres of Xenopus laevis and Zebrafish, where cells are extremely large. Mitotic and interphase asters differ markedly in size, and only interphase asters span the cell. Growth of interphase asters occurs by a mechanism that allows microtubule density at the aster periphery to remain approximately constant as radius increases. We discuss models for aster growth, and favor a branching nucleation process. Neighboring asters that grow into each other interact to block further growth at the shared boundary. We compare the morphology of interaction zones formed between pairs of asters that grow out from the poles of the same mitotic spindle (sister asters) and between pairs not related by mitosis (non-sister asters) that meet following polyspermic fertilization. We argue growing asters recognize each other by interaction between antiparallel microtubules at the mutual boundary, and discuss models for molecular organization of interaction zones. Finally, we discuss models for how asters, and the centrosomes within them, are positioned by dynein-mediated pulling forces so as to generate stereotyped cleavage patterns. Studying these problems in extremely large cells is starting to reveal how general principles of cell organization scale with cell size.
雷·拉帕波特（Ray Rappaport）多年来一直致力于研究微管星体及其诱导分裂沟的方式。在这里，我们回顾了最近在非洲爪蟾（Xenopus laevis）和斑马鱼（Zebrafish）的受精卵和早期卵裂球中星体结构和动力学的研究进展，这些细胞的体积非常大。有丝分裂和间期星体在大小上有明显的差异，只有间期星体跨越整个细胞。间期星体的生长通过一种机制发生，该机制允许星体边缘的微管密度在半径增加的情况下保持大致恒定。我们讨论了星体生长的模型，并倾向于分支成核过程。相邻的星体在相互生长时会相互作用，阻止在共享边界处进一步生长。我们比较了从同一有丝分裂纺锤体的两极（姐妹星体）生长出来的两对星体之间以及在多精受精后相遇的不相关的两对星体（非姐妹星体）之间形成的相互作用区的形态。我们认为，生长中的星体通过相互作用区的平行微管之间的相互作用来识别彼此，并讨论了相互作用区的分子组织模型。最后，我们讨论了星体以及其中的中心体如何通过 dynein 介导的拉力来定位，从而产生典型的分裂模式。在非常大的细胞中研究这些问题，开始揭示细胞组织的一般原则如何与细胞大小相适应。
